This invention relates to a mattress covering sewing machine. A mattress consists basically of a cushion constituting an inner structure and an outer covering which three-dimensionally covers the cushion. The outer covering consists of a pair of upper and lower covers and a side cover i.e. a gusset which covers the side surface of the cushion. The upper cover and gusset are sewn together into a box-like configuration. The cushion is wrapped with the box-like configuration. The cushion is wrapped with the box-like cover and the lower cover is placed under the open end of the box-like cover and the corresponding outer marginal edge portions of the adjacent covers are sewn together to form a mattress. In this case, it is required that the outer covering be tightly wrapped around the entire periphery of the cushion without leaving any sagging. For this reason, the outer cover is made somewhat smaller in size than the outer shape of the cushion. When the outer marginal edge portions of the adjacent outer covers are finally sewn together, it is necessary that the cushion be storongly compressed with the elbow of the sewer so that the outer marginal edge portions of the adjacent outer covers are sewn together to form a mattress. For this reason, a high skill is required on the side of the sewer and it is impossible to uniformly compress the cushion with the elbow of the sewer and, in consequence, mass produce uniform mattresses.